Pattern-imprinted paving: a cost-effective alternative

ANDY LUXTON , PICS LTD

Introduction
Concrete is universally the most commonly used construction material for foundations, reinforced frames for buildings, bridge decks, retaining walls, roof tiles, insitu and precast floors. However, although many countries have also adopted it for in-situ paving, this is not the case in the UK. The reason is that, despite concrete's remarkable durability, a number of architects, designers and professional bodies such as English Heritage still regard it as a last resort - the cheapest and least desirable form of paving.

Some 14 years ago, while employed by a ready-mixed concrete company, I visited a small contractor who was laying a strange type of concrete - pattern-imprinted concrete. Since then, the industry has developed to a point where 1,OOO,OOOm2 are now installed each year.

The system combines colour surface hardeners, release agents and acrylic sealers with three-dimensional texturing tools to create remarkable designs. Many designs replicate bricks, stone, slate, tile and even timber decking in a range of over 40 different colour combinations. This achieves the goal of a decorative and aesthetically pleasing surface treatment with a versatile and durable insitu concrete.

To achieve the workability and consistency required to apply the colour hardeners and imprint the design, a very specific concrete mix is required. It is designed around a minimum cement content of 320-375kg/m3 dependent on ambient temperature and other conditions. This allows the initial set of the concrete to be controlled while ensuring that final strength is achieved at 30-35N/mm2. A 10mm aggregate is ideal, but 20mm may be used together with a 55% fines content for workability. As with any external concrete paving, 5% air is entrained with 6mm monofilament fibre reinforcement to reduce the risk of surface shrinkage during curing. Water content is always an important issue with any mix, and a 5Omm slump is recommended, normally rising to 75mm for pumping.

The main aim of any decorative concrete system should not be to imitate another paving material. Instead, it is a way of paving in a continuous slab to accommodate types of traffic or to suit environments where unit paving is vulnerable, at the same time minimising future maintenance demands. Three typical projects will help explain why specifiers are choosing pattern-imprinted concrete over other materials.

Rue de Luxembourg, Brussels
In the heart of Brussels, between the Royal Palace and the European Parliament, Rue de Luxembourg is a vital access point for pedestrians and buses reaching the city centre. For some time the cobbles had been problematic, with the constant bus traffic causing them to move and loosen. The answer was to lay a decorative concrete to ensure durability, yet reflect the style of the surrounding cobbled streets. An average 300mm of lean-mixed concrete was laid on geotextile with a 50mm bituminous concrete layer laid on top of this foundation. The imprinted concrete surface was then installed to an average depth of 230mm and finished in a deep-joint cobble-stone design. In total, 3,700m2 of imprinted paving were installed by a five-man team working 20 continuous days, showing a substantial saving over unit pavers. The completed project has so impressed the client that further projects have been specified.

Carden Park Hotel and Country Club, Chester
Imprinted concrete paving is not only suitable for external projects, but can also be used as a traditional screed in place of tiles or carpet. In 1997, the Carden Park Estate was renovated and developed to create a new Hotel and Country Club. The interior design team specified a stone appearance for the foyer area without the associated cleaning problems. From various proposals, they chose Picspave imprinted paving in slate green with a basket-weave stone texture. An area of
280m2 was completed in seven days and has proved an excellent solution. Other internal works include museums, restaurants and residential projects.

Southport Promenade
The north-west coast of Britain is renowned for its winter storms and is not the ideal location for laying a decorative concrete project in February. However, this was the challenge offered by Sefton Metropolitan Borough Council for the installation of their public arts project, part of the new sea wall protection scheme. Behind the new sea wall, the five areas of paving designed by the Partnership Art Organisation represent the phases of the moon. The curves and angles of the designs were very tight and therefore difficult to transfer from the drawing to site. Complex shuttering was assembled to form a large number of individual concrete pours, each of which was coloured and textured individually. Colour hardeners were used to form sweeping swathes of colour blending into others. The surface was then textured with natural sponges and sealed after curing with an acrylic sealer. Templates were produced to imprint 'shooting star' symbols into the concrete in a number of areas. Altogether, some 1500m2 of paving were installed during three site visits in the most inhospitable weather, with very high winds.

Conclusion
Concrete may not be the most fashionable building material, but it can form the base material for any number of finishes and should be more readily considered by specifiers, designers and architects. Pattern imprinted concrete offers a host of opportunities, without the need to import materials from China, the Far East and mainland Europe.